Mass-market optical data storage is familiar in the form of the well-known CD-ROM optical disc format. A historical review and technical description of these and other storage media can be found in The PC Technology Guide, itself available on CD-ROM, and at http://www.pctechguide.com/10dvd.htm. DVD expands upon the capabilities of CD-ROM, in terms of its nominal (“1×”) data rate, as well as its data capacity. A DVD/CD decoder is described for example in WO-A-99/48097. Integrated circuits (ICs) incorporating suitable circuitry are available from Philips Semiconductors, for example product SAA7335.
Current CD-ROM drives reproduce the stored data at many times the nominal data rate for which the CD medium was designed, and are designated “24×”, “32×”, “48×” accordingly. The simplest way to increase data rate is to increase the speed at which the disc spins over the optical pick-up. The race to build ever faster optical disc drives continues, but physical limitations in disc tensile strength, and in the power consumption has put a limit on the spin speed at between 200 and 400 Hz. For DVD, this equates to an “x” rating of 20-40×.
One solution to the problem of delivering an ever higher data rate from a standard optical disc without increasing the spin speed is the so-called “Multi-Beam” approach. In a Multi-Beam system the read-back laser beam is split in an optical pick-up unit (OPU) into a number of separate beams focused on adjacent radial tracks. With an OPU of N beams, the data can be read (in principle) at N times the rate, for a given speed of rotation of the disc. A multi-beam architecture for CD systems is described in WO-A-98/037555 (Zen Research). This description assumes that multi-track read out is achieved using a single laser combined with a diffraction grating which are arranged such that N neighboring tracks are read in parallel. This means that the IC must have N data inputs and is capable of processing N data streams simultaneously. More elaborate arrangements with separate read heads are also known, in the context of CD readers, for example from U.S. Pat. No. 5,465,244 (Kobayashi/Toshiba).
The Multi-Beam approach also brings the possibility of reducing the power consumption of a drive if data rate is not an issue, as for a given data throughput, the disc spin speed can be proportionally reduced by a factor related to the number of beams. This approach gives significant power consumption benefits for portable equipment, as the power consumption of such drives is dominated by the power dissipated by the spindle motor and drivers.
A DVD disc was designed to be read in a linear fashion, and although random access through jumps is part of the system design, the intention is that data is normally read as a stream, i.e. Data is continuous on the track from the start to the end of a file. All the data on a DVD (as is a CD) is arranged on a single spiral track running from the inner disc radius to the outer. If a multitrack approach is used it can be easily seen that the data being read by the individual pick-ups is effectively data from the same linear stream, but temporally shifted by one disc rotation, this means for an N beam system one disc rotation will yield N rotations worth of data. The data from the various pickups needs to be reassembled into one linear data set before passing on to the host application. After one rotation of the disc, the read-back head then needs to jump out N grooves to start acquiring the next consecutive block of data.
A problem arises when one tries to extend the multi-beam concept from CD to DVD drives, however. Like a CD-ROM, the data on a DVD disc is organized into 2 Kbytes sectors. However, while the CD system incorporates essentially a continuous linear code for error protection, DVD adopts a 2-dimensional block-based scheme, in which the sectors are organized for error protection purposes into larger blocks called ECC frames. Each ECC frame occupies a substantial portion of one rotation of the disc, especially at the inner radius of the spiral, and frame boundaries on adjacent tracks are not aligned. Known decoders for DVD assume that a complete ECC frame is received from start to finish, in order to decode and correct the data. Accordingly, since each OPU beam will in general wait some time before it encounters the beginning of a new ECC frame, the theoretical benefit of multi-beam read-out cannot be realized with known DVD decoders. Of course, the above problems may be solved in the simple way of just replicating a single channel system N times then re-combining the data using software and a large data buffer, but this solution leads to multiple instances of large buffer memory, and in a 200 Hz (20× rotation speed system) will impose serious demands on the bandwidth of the buffer memories. A particular problem is that large quantities of memory and complex, high-performance logic circuitry cannot readily be provided together on a single low-cost IC.